The general procedure in fabricating permanent restorations in current dental implant practice is as follows. The fixture is the component which is surgically placed into the jawbone; this is often accomplished by a periodontist or oral surgeon. After a healing period of 3-6 months, during which a process of bone growth around the fixture called osseointegration occurs, the implant is exposed. A general dentist or prosthodontist then performs the restoration, which involves placement of an abutment of a specific size and shape over the fixture and securing the same by means of a bolt threaded into a cavity in the fixture. The implant distal surface contains a flat, polished outer ledge and a central hex which is then engaged by a tool during placement. Also indicated by the prior art is a submucosal healing cap and a transmucosal healing cap which prevent the fixture from becoming infiltrated with tissue from the gingiva and/or bone. In addition to keeping tissue out of the fixture the healing cap establishes a sulcus or opening above the fixture to allow placement of the second component, the abutment.
The abutment is secured into the threaded cavity of the fixture by a titanium bolt, called the abutment retaining screw. The prosthesis is the third component of the system; this element is fabricated of cast gold alloy and porcelain. However, since machined parts have greater accuracy than cast parts, the prosthesis is commonly cast to a machined component called the gold cylinder. This gold cylinder, as part of the completed prosthesis, is fastened to a threaded cavity in the abutment retaining screw by the gold screw. This gold screw, being smaller and of weaker alloy than the titanium abutment retaining screw, should normally be the first to fracture if excessive force is encountered.
One modification of the above system is to attach the prosthesis directly to the fixture without any intervening abutment. This method is termed the UCLA modification and uses only the large titanium screw.
A further variation is to cement a restoration to implants in the same manner as is done in conventional fixed bridges on natural teeth. In this case, a tapered abutment without threads, often referred to as a cementable abutment, is fastened to the fixture with a large titanium abutment retaining screw. Thus, this method also has only one screw in the system.
To summarize, implant dentistry relies upon screws to fasten together component stacks. These stacks consist of the fixture, abutment, and prosthesis (commonly fabricated around a machined gold cylinder). A second possibility consists of just the fixture and prosthesis (UCLA), eliminating the abutment for reasons of esthetics, angulation, etc. A third possibility consists of an abutment to which the prosthesis is cemented.
Screws used as fasteners can loosen when subjected to cyclic or vibratory loads. Such loads certainly occur in the mouth. This loosening can be viewed more accurately as slippage of the entire joint, which consists of the two components involved and their fastening screw.
Consequences of screw loosening in the general case are:
1) Repeated loosening of the restoration. If the frequency is months or weeks the loosening becomes unacceptable to both the patient and the dentist.
2) Gold screw or abutment retaining screw bending.
3) Gold screw or abutment retaining screw fracture.
While these problems with the fastener do not occur in the majority of implant cases, their frequency is sufficient that the causes are being actively investigated.
In some instances, the system is overloaded, for instance by placing too few implants for the number of teeth being replaced. In these cases, gold screw fracture or bending is the most preferable outcome, because it gives the clinician warning that the system is being overloaded. Gold screw bending or fracture is less of a problem than abutment retaining screw bending or fracture, since the gold screw is most easily retrievable. Abutment retaining screw fracture can be dealt with by removing the fragment of the abutment retaining screw contained within the threaded cavity in the fixture. This procedure is usually difficult and can even irreversibly damage the fixture. Fixture fracture or failure of osseointegration is the least desirable outcome of overload, as these imply loss of the fixture. If the gold screw breaks, there is still time to reconsider the placement of fixtures and the design of the prosthesis and make corrections, perhaps by adding more fixtures.
Screws are also known to loosen in many cases that are well designed, have sufficient fixtures and appear to fit very accurately. These instances of screw loosening are due to vibration, defined as low but highly repetitive forces on the joint. Vibration has a tendency to loosen bolts and screws. It has been postulated that very small movements of the implant prosthesis, termed micromovements, occur in response to vibration and increase the chance of screw loosening. This is at least part of the motivation to cement restorations; note, however, that there is still a screw in the cementable abutment.
When a screw is tightened, a tensile force, termed the PRELOAD, is built up in the screw, mainly between the head and the first few threads. This preload is what holds the components of an implant component stack together. The screw is placed in tension, and the components fastened by the screw are placed in compression. Preload also prevents loosening of the screw. The preload should be as high as possible (for a given tensile strength of the screw material) and should fluctuate as little as possible to prevent loosening.
Occlusal forces from chewing, speaking, bruxing, etc. (which can be viewed as vibrations) load the prosthesis and place forces on abutment retaining screws and gold screws which may result in loosening of the screws. If the screw loosens, the preload is decreased or lost, the screw joint opens up, and the screw will then loosen further, bend or break. Once permanent deformation takes place, either through wear-and-tear effects or through gross bending, there is nothing to prevent the screw from loosening.
Additional effects act on screws to reduce preload. When any implant screw is tightened for the first time, contact between its threads and the screw channel walls only occurs on microscopic areas of roughness. Plastic flow of these initial contact points occurs and reduces preload. This phenomenon is called embedment relaxation or settling effects. Thus, the torque used to place a retaining screw initially is greater than that required to remove it.
One proposed solution to screw loosening is using high torque or torque within a certain range in the placement of the various retaining screws. However, what constitutes the proper torque has not been determined by controlled scientific investigation. Also, the torque required to loosen an implant screw is less than that used to tighten it, due to settling effects and wear-and-tear effects on the screw threads. High initial torque may not prevent screw loosening months or years after placement, due to wear-and-tear effects and the cyclic loading that occurs in the mouth. Even if an ideal initial torque could be determined, it has been shown that dentists vary widely in their ability to place a screw within a specified torque range. Mechanical torque drivers are necessary to achieve consistency, but this application only relates to INITIAL torque values, not those achieved after settling effects and cyclic loading. Very high torque may create torsional stress on the screw beyond safe limits, leading to permanent deformation and fracture. Thus, placing screws with high torque is not an ideal solution to the problem of retaining screw loosening in well-designed implant cases.
Spring washers of the helical, split-lock type (hereafter simply called "lockwashers") and/or Belleville washers work on many levels to help prevent screw loosening. With respect to helical washers the descriptive material in my co-pending application Ser. No. 159,326 is hereby incorporated by reference.
A spring washer placed under the screw head maintains a constant tension in the screw, decreasing the chance of loosening under cyclic or vibratory loads. The spring washer acts as a damping mechanism for micromovement, preventing transmission of that movement into the screw.
Washers act to distribute loads and provide a surface for uniform torque control. By increasing the preload and the clamping forces, spring washers may make the screw joint more resistant to opening up and subsequently bending or fracturing.
Some of the kinetic energy of screw tightening is converted into potential energy in the spring of the washer; thus, spring washers store energy. This energy adds to the preload. Another way in which washers add to preload is more subtle. For hard metal screws and screw channels, up to 90% of the applied torque is used to overcome the friction forces caused by the screw threads and under the screw head. Washers represent dry lubrication. Reducing the coefficient of friction of the screw in its channel and/or under the screw head acts, according to the principles of operation of fasteners, to increase the preload of the screw for a given applied torque. Consequently, the possibility of loosening is decreased significantly. The increased preload also reduces the working stresses in the components held together by the screw, decreasing the possibility of fatigue failure due to cyclic stress.
The most favorable location of the spring washer in order to achieve this effect is on the screw journal just below the head. Also, the principles of operation of fasteners show that the preload is inversely related to collar size; thus having the screw head bear on the spring washer at the smallest possible diameter will give the greatest increase in preload. Note that the screw head can be of large diameter; the diameter of the bearing surface of the washer against the screw head is the factor which determines the preload. Belleville washers with their conical shape can accomplish this effect if they are placed on a screw at the journal, just under the head.
The effect of reducing the coefficient of friction and/or the collar radius of the screw head is to increase the preload for a given torque. This avoids the problems of extremely high torque placement of screws, which places high torsional stress on the screw and weakens it. In other words, for a given torque, one can have higher preload with a washer.
One final way of thinking about lockwashers is to examine what % of a full turn of a retaining screw it takes to dump all of the preload out of the system. In current implant practice, a very small turn of the screw, perhaps as little as 1/32 of a turn, would be sufficient to eliminate most of the preload. With a lockwasher of appropriate torsional stiffness, a significant fraction of preload could be maintained even if the screw was backed off 1/4 or 1/2 of a turn. This arrangement would allow more leeway, in terms of time, to intervene before loosening and damage took place.
One current example of an anti-slippage mechanism in dental implants is filed as serial number for patent pending Ser. No. 159326, filing date Nov. 30, 1993. This patent application shows a split-lock type lockwasher in various modifications placed between the healing cap and the fixture. An opening in the gingiva (gum) is created surgically and preserved by use of the healing cap body. The cap is intended to pass through the gingiva to the outer surface of the surrounding gingiva. The underside of the cap is shaped in various modifications to provide a cavity or flat surface which accepts a lockwasher. The lockwasher is of the split-lock, helical type. The cap is installed on the implant by threading a separate screw into the threaded base of the implant with the lockwasher in between until the proximal surface of the cap is in contact with the washer, which is in contact with the distal surface of the implant. The goal of healing caps in general is to shield the upper surface of the implant from overgrowth of gingival tissue and at the same time maintain an opening through the gingival tissue which overlies the implant. The novelty of this invention is to provide resistance to slippage so that the healing cap cannot loosen in response to muscular movements in the mouth.
It is an object of the current invention to design a gold screw/gold cylinder assembly and an abutment retaining screw/abutment assembly that will not loosen when used in a well-designed implant restoration.
It is a further object of this invention to prevent screw bending and/or fracture secondary to loosening.
It is still further an object of this invention to accomplish the first two objects for many if not all of the current commercially available implant component systems without major modifications to those systems.